Spectral imagers measure the reflectance energy spectrum of materials within a field of view. The reflectance, reflectivity, or reflectance coefficient is a number between 0 and 1 that determines the proportion of light at a given wavelength that is reflected by a particular material (as opposed to being absorbed). Leaves, for example, reflect photons in the green portion of the visible spectrum at a much greater proportion than photons in the red or blue portion. The reflectance spectrum of a material contains the reflectance coefficient for that material at each wavelength within a relevant range. The reflectance spectrum is valuable because it contains information about a material's chemical makeup and constituents. This type of information is useful in several domains, including, but not limited to, agriculture, geology, astronomy, defense, and intelligence applications.
Spectral imagers record energy from the field of view at a multitude of spatial picture elements (pixels). The recorded energy is radiance, which contains the material reflectance information as well as other sources of information, including illumination conditions and atmospheric conditions. The raw collected imagery can be converted to reflectance values at each of multiple spectral bands per pixel. The data set generated by spectral imagers is a three-dimensional “cube” having two spatial dimensions and one spectral dimension.
Hyperspectral imagers are a class of spectral imagers that can generate a spectral cube with relatively higher resolution in the spectral dimension. Hyperspectral imaging is typically defined by having tens to hundreds of discrete spectral bands within a certain wavelength region. Conventional hyperspectral imagers may rely on either sequentially capturing a series of spatial images, each spatial image representing a certain spectral component (“pushbroom imagers”), or sequentially capturing a series of spectral profiles, each spectral profile representing a certain spatial portion (“staring imagers”). Generally, in both pushbroom and staring imagers, one or more components, such as an aperture, mirror, or filter, is physically moved to perform a scan over either the spectral or spatial dimensions. Precise control of the movement of these components is important to generation of high spatial and spectral resolution.
Conventional hyperspectral imagers are large and complex devices that are unsuitable for hand-held or portable applications and are too costly for many applications.